Generally speaking, as the DC voltages needed by the motherboard and the peripheral devices (such as the floppy disc drive, hard disc drive, optical disc drive, electric fan, etc.) of the computer are variant, the power supply, which provides the DC power for the computer system, has to output at least two different levels of DC power to satisfy the variant voltage demands of various devices in the computer system.
Recently, as the computer's working speed demanded by the user becomes higher and higher, the electric power needed by CPU and the peripheral devices in the computer system grows responsively; therefore, the power supply of the computer system has to provide more power output. However, owing to the variation of the computer design, the power of the output ports have an energy-limiting level (maximum output power Max VA) in some systems; thus, these output ports need the design of multiple sets of current-limiting compare units 31 to satisfy those users who demand high-grade safety standard. In general, for a power supply, there is an energy hazard standard, such as in SSI or UL, which requires that the highest power output by the power supply should not exceed 240 VA, in order to secure the user's safety.
The computer systems shift rapidly, and the power demand is higher and higher. For the user who just persists in the high-grade safety standard, the only way to satisfy the power demand is to increase more and more current-limiting compare units 31 so as to raise the power output. However, this is not a good method in the long run as the more current-limiting compare units 31, the harder the design of the power supply, and the cost of the power supply will be further raised as the users who require the high-grade safety standard are still the fewer. Therefore, if the power demand of the computer system keeps on rising, the system manufacturer will choose a lower-grade safety standard to design the system, i.e. to utilize less or even only one current-limiting compare unit 31 in the system, and the case will return to the starting point. The current power supply having the current-limiting compare units 31 in the market will be described below with the drawing and its connecting and operating manner.
Refer to FIG. 1 a block diagram of the conventional power supply with a single current-limiting compare unit 31. As shown in the drawing, the conventional power supply comprises: a power output circuit 10, which provides the power, wherein a constant current source 32 and a reference resistance 34 generate a current-limiting reference value Va and a sampling circuit having a level resistance 33 generates a current-limiting compare value Vb; the current-limiting reference value Va and the current-limiting compare value Vb are separately coupled to two input terminals of the compare unit 31 to compare each other in order to determine whether the compare unit 31 should output normal-state signal or false-state signal to the power output circuit 10. If the current-limiting compare value Vb is equal to or less than the current-limiting reference value Va, the compare unit 31 outputs a high electric signal to enable the power output circuit 10 of the power supply to enter a shut-down state to shut off the power of the computer. Conversely, if the current-limiting compare value Vb is more than the current-limiting reference value Va, the compare unit 31 outputs a low electric signal to the power supply to enable the power supply to operate normally, and thus the user can continue to use the computer. it is to be noted that in this drawing, the compare unit 31 takes the integrated current-limiting compare value Vb of all the power output to multiple loads 40 to undertake the comparison.
As shown in FIG. 2, another specification of power supply adopts multiple sets of compare units 31, wherein each individual set of compare unit 31 operates in the same mode as that in FIG. 1. This specification of power supply is designed according to the highest-grade design standard for the upmost user safety level, and even only a single set of compare unit 31 overloaded, the power supply enters a shut-down state to shut off the power of the computer in order to protect the user's safety; however, far those persons skilled in the computer technology, this way will limit the configuration of the devices with the loads 40.
It is to be known from those described above that there are two kinds of compare units 31 of the power supply in the current market; one is the multiple-set compare unit 31 for the general computer user, and the other is the single-set compare unit 31 for the computer technician; thus, the manufacturer of the power supply has to design variant specifications of power supplies to satisfy the need of various users, which not only makes the products lowly compatible and the fabrication harder, but also consumes higher cost. Thus, the problems mentioned above are those the manufacturer desires to solve.